1. Field of the Invention
The present invention relates to a warm white fluorescent lamp which produces light in the warm white region.
2. Description of the Related Art
Three band fluorescent lamps having high efficacy and high color rendering properties are widely used as general lighting lamps. The three band fluorescent lamp is one that produces light predominantly in three wavelength regions where the human eye is most sensitive to color, that is, blue at about 450 nm, green at about 540 nm, and red at about 610 nm, and thus achieves increased color rendition without reducing luminous flux. Three band fluorescent lamps with different light colors are produced by varying the energies in the three wavelength regions.
For the past few years, three band fluorescent lamps designed to produce light that simulates the incandescent lamp color to create a warm and relaxed atmosphere (hereinafter called three band warm white fluorescent lamps) have been drawing much attention and have been finding application for indoor lighting. The warm white resembles the light color of an incandescent lamp, and since the three band warm white fluorescent lamp is superior in efficacy to the incandescent lamp, this type of fluorescent lamp has the potential of replacing the incandescent lamp.
Though the light colors are similar, the three band warm white fluorescent lamp is inferior in color rendition to the incandescent lamp in the following respect.
The incandescent lamp has the color characteristic that not only enhances a warm and relaxed atmosphere but has the effect of brining out the beauty of colored objects and creating a high-quality atmosphere.
The three band warm white fluorescent lamp, on the other hand, is inferior to the incandescent lamp in terms of object color appearance, especially the appearance of red and reddish object colors. The previous three band warm white fluorescent lamp thus has had the problem that its color rendition is clearly inferior to that of the incandescent lamp when it comes to the high-quality atmosphere associated with the color appearance that the incandescent lamp can provide. As a piece of information, the appearance of green and greenish object colors is comparable to or better than that of the incandescent lamp.
In view of the inferior appearance of red and reddish object colors associated with the previous three band warm white fluorescent lamp, it is an object of the present invention to provide a three band warm white fluorescent lamp that achieves red and reddish color appearance comparable to or better than that of the incandescent lamp and green and greenish color appearance comparable to or better than that of the incandescent lamp, and yet has higher efficacy than that of the incandescent lamp.
Regarding color appearance under illuminating light, a method known as the "method of evaluating the fidelity of color appearance" is used to quantitatively evaluate the color rendering properties of a light source. This method, specified in JIS Z 8726-1990 "Method of Specifying Color Rendering Properties of Light Sources", is intended to quantitatively evaluate how faithfully the lamp under test is rendering the target color in comparison with the color rendered under a reference illuminant. JIS Z 8726-1990" "Method of Specifying Color Rendering Properties of Light Sources" is essentially regulated in accordance with the method set forth in the CIE Pub.13.2 "Method of Measuring and Specifying Color Rendering Properties of Light Sources(1974)". The value is expressed in terms of the General Color Rendering Index Ra. Nowadays, development of fluorescent lamps are directed toward increasing the General Color Rendering Index Ra and improving luminous efficacy.
On the other hand, besides the method of evaluating the fidelity of color appearance, work on a "method of evaluating the desirability of color appearance" has been proceeding in recent years. This method is intended to quantitatively evaluate whether the color shift caused by the light source under test, as compared with the color observed under the reference illuminant, is in the desirable direction or undesirable direction. This evaluation method shows that if there is a color shift when evaluated in terms of the fidelity described above, if it is a shift in the desirable direction it will bring a better result in terms of color appearance when seen by humans.
Regarding the method of evaluating the desirability of color appearance, by noting the visual clarity (visual clarity effect) associated with the color rendering properties of a light source, it has been found that the degree of vividity of object color appearance can be evaluated using a visual clarity index developed from the concept of visual clarity, as described in such publications as "Hashimoto et al., Visual Clarity and Feeling of Contrast, Color Research and Application, 19, 3, June (1994)," "Hashimoto et al., Method of Evaluating the Color Rendering Properties of a Light Source Based on Visual Clarity, J. of the Illuminating Engineering Institute of Japan, Vol. 79, No. 11, 1995," and "Japanese Patent Unexamined Publication NO. 6-180248 (Japanese Patent Application No. 4-333919).
Accordingly, in developing a warm white fluorescent lamp that can produce the same beautiful, high-quality atmosphere as obtained from the object color appearance of the incandescent lamp, if the color appearance of objects important for the visual clarity effect of the light source, i.e., the red of food and the green of tree leaves, can be made comparable to or better than the color appearance of the incandescent lamp by using the method of evaluating the desirability of color appearance, then it will become possible to create a high-quality atmosphere by effectively eliciting the beauty only of the object colors that are important for the enhancement of the high-quality effect.
Stated another way, the most important issue is to determine the color characteristic range within which the color appearance of objects important for the visual clarity effect of the light source, i.e., the red of food and the green of tree leaves, is substantially comparable to or better than the color appearance of such objects under the incandescent lamp by using the method of evaluating the desirability of color appearance.